1. Field of the Invention
This invention relates to a process for the conversion of isobutyric acid to methacrylic acid including the like conversion of a lower alkyl ester of isobutyric acid.
2. Description of the Prior Art
There is considerable prior art directed to the oxydehydrogenation of the lower saturated mono-carboxylic acids to prepare the corresponding .alpha.,.beta.-ethylenically unsaturated acids. The initial work reported in this area was that of thermally effecting the indicated oxydehydrogenation by the vapor phase reaction of the acid substrate with iodine and oxygen. This approach has not attracted much attention as a potentially viable way for commercially implementing the underlying reaction. This is understandably so inasmuch as iodine is costly, exhibits extreme corrosivity properties and poses considerable problems in realizing complete recovery of the comparatively large amounts thereof required in the process.
As the subsequent prior art picture amply points up, the heterogeneous catalytic method for effecting the oxydehydrogenation reaction is viewed as being much more attractive from the standpoint of potential commercial applicability. In the main, the more recent relevant prior art activities have centered on the use of two types of catalyst compositions for this purpose. One type includes generally the heteropoly-acids, typically representative of which 12-molybdophosphate optionally including vanadium and/or tungsten elements in a like structural arrangement. The other type catalyst includes those systems having in common a calcined iron phosphate matrix.
Iron phosphate subjected to calcination exists in a plurality of crystalline phases or species. While it is believed that the oxidation/reduction coupled involved in the underlying reaction is attributable to the iron phosphate, which species is or are catalytically active has not been identified. There is, however, evidence that the presence of an extrinsic metal component in the preparation serves to facilitate the formation of the catalytically active species. For example, U.S. Pat. No. 3,948,959 notably teaches that an alkali or alkaline earth metal as the extrinsic metal component is effective for this purpose. The present invention accordingly represents a furtherance of this particular aspect of the current state of the art.